1. Field of the Invention
This invention relates to the field of communications, and in particular to a quadrature transceiver that includes pre-distortion and post-distortion compensation for frequency-dependent I/Q channel mismatch.
2. Description of Related Art
The use of quadrature modulation and demodulation is a common communication technique for communicating digital data as a stream of two-bit symbols. A first stream corresponding to one of the bits of the symbol is modulated by an xe2x80x9cin-phasexe2x80x9d (I) oscillation signal, and a second stream corresponding to the other bit of the symbol is modulated by a xe2x80x9cquadrature-phasexe2x80x9d (Q) oscillation signal that is ninety degrees out of phase from the in-phase (I) oscillation signal. The I and Q modulated signals are combined to form a composite signal for transmission. The orthogonal nature of the modulation allows for a reliable demodulation of individual I and Q modulated bit streams at a receiving system.
IEEE 802.11a/g specifies an Orthogonal Frequency Division Multiplex (OFDM) scheme that employs a combination of frequency division multiplexing and quadrature modulation and demodulation to effect high-speed wireless data transfer. At the OFDM transmitter, the outputs of a plurality of quadrature modulation systems are frequency-division-multiplexed for transmission to a corresponding OFDM receiver. As in all quadrature modulation and demodulation systems, OFDM systems are sensitive to phase shifts that cause the I and Q modulated signals to become non-orthogonal, commonly termed xe2x80x9cI/Q channel mismatchxe2x80x9d.
U.S. Pat. No. 6,298,035 xe2x80x9cESTIMATION OF TWO PROPAGATION CHANNELS IN OFDMxe2x80x9d, issued Oct. 2, 2001 to Juha Heiskala, incorporated by reference herein, provides an overview of the principles of OFDM modulation and demodulation, and discloses a method of estimating the frequency response of each channel by transmitting select training symbols between two transceivers. This method particularly addresses the frequency-dependent effects caused by multipath fading and interference, and provides an adaptive solution based on actual transmissions from one transceiver to another.
It is an object of this invention to provide a system and method that minimizes I/Q channel mismatch. It is a further object of this invention to provide a system and method for minimizing I/Q mismatch across a range of frequencies. It is a further object of this invention to provide a system and method that effects autonomous I/Q mismatch compensation within a single transceiver.
These objects, and others, are achieved by providing a combination of pre-distortion and post-distortion processes that compensate for errors in I/Q channel orthogonality. The pre-distortion and post-distortion processes are calibrated to compensate for these errors at a variety of frequencies across a frequency span, thereby providing frequency-dependent compensation for I/Q channel mismatch. Pre-distortion calibration is effected by coupling the filtered analog I/Q modulated signals from the transmitter of a wireless transceiver directly to the analog-to-digital converters of the receiver of the wireless transceiver. Post-distortion calibration is effected by coupling the analog I/Q modulated signals from the transmitter directly to the channel filters that precede the analog-to-digital converters of the receiver.